In a compressor such as a wobble plate type compressor, lubrication for the driving mechanism in the crank chamber is generally supplied by blow-by gas which is mixed with lubricating oil in a mist state. Also, in a compressor which is provided with a variable capacity mechanism to change the compressor volume by changing the inclined angle of a cam rotor, it is necessary to control the pressure in the crank chamber to change the compressor volume. The crank chamber communicates with the suction chamber through a passageway, and communication therebetween is controlled by the opening and closing of a valve device. Therefore, the blow-by gas sometimes returns to the suction chamber through the passageway to control the pressure in the crank chamber.
As mentioned above, blow-by gas is very important for operating the compressor. Therefore, the amount of blow-by gas leaked into the crank chamber should be maintained at a stable level. Accordingly, a conventional piston ring fitted on an outer surface of a piston is provided with a plurality of grooves on its outer peripheral surface to communicate between the interior of the crank chamber and the inside of the piston cylinders to secure the amount of blow-by gas.
The grooves of the piston ring are generally formed by a cutting or turning tool. However, since a plurality of grooves should be formed on a single piston ring, manufacturing time increases and precision decreases.